In today's NY Times, seasonal weather forecaster Judah Cohen explains (here) how the Earth system can warm at the same time that winters in the Northern Hemisphere (NH) become cooler. Cohen also points out some problems in long-term weather forecasts and the Global Circulation Models (GCMs) used to predict climate change.Here's the causal explanation (see the directed graph on the right): Global warming increases Arctic sea ice melt. As the sea ice melts, more moisture is released into the atmosphere. More moisture means that there will be more snow in Siberia. As snow cover increases, more energy is reflected back to space (the Earth's albedo or "whiteness" increases and white objects reflect more energy).
As more energy is reflected back to space, an Arctic cold air dome forms over Siberia. The large dome of cold air shifts the jet stream from its normal West-to-East direction to a more North-South oscillation. As the Jet stream moves from North to South it acquires Southern moisture and pulls down Northern Canadian cold air (in the US).
The predictions from Cohen's model for the U.S. (displayed at right) show that the Northeastern U.S. was predicted to be colder than usual while the Southern U.S. was predicted to be warmer. The actual trends (lower graphic) were very close to the model's predictions.
Long-term weather forecasts are largely based on the El Nino/La Nina-Southern Oscillation (ENSO). Warming or cooling of the tropical Pacific Ocean on a five-year cycle cause weather disturbances for the entire planet. Since the oscillation is quasi-deterministic and since the effects of the oscillation on weather are known from historical data, long-term weather prediction is possible. The current long-term forecasts do not take into account Siberian snow fall and neither do the GCMs that are used to predict global warming. We can expect some improvements in forecasting and climate change predictions as ENSO and the Arctic snow cycle are better understood.
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